It is known in the art to make liquid containers comprising a pallet, a protective cage mounted to the pallet, and a plastic container within the cage, for containing the liquid.
The cage is composed of a plurality of vertical and horizontal tubular metal bars, welded together at their intersections. The cage has the shape of a parallelepiped and a size that allows it to be fitted on the inner plastic container.
The most common capacity of these containers is about 1000 liters.
During transportation, the pallet container is subjected to mechanical stresses, for instance caused by road bumps or curves or the gap between rails of a railroad along which the container-carrying vehicle runs.
Particularly, as a liquid is being carried, it is subjected to movements and vibrations which cause cyclic and non-cyclic stresses on the side walls of the plastic container and hence on the walls of the cage.
In view of these stresses, the standards that govern carriage of dangerous goods (which is the main application of these containers) require manufacturers to perform a number of tests on the containers before introducing them into the market.
One of these tests requires the container supporting surface to be vibrated with a 25 mm oscillation amplitude and at such a frequency as to allow the passage of a 1.6 mm thick metal blade between the bottom of the container and the corresponding support surface. According to this standard the blade must be 50 mm wide and be able to be inserted between the pallet and the support surface through at least 100 mm. The container must be filled to not less than 98% of its maximum capacity.
According to the type of container, the testing frequency may be of the order of 150-230 rpm, e.g. about 180-190 rpm.
Under vibrational stresses as defined above, the pallet container was found to be sometimes damaged or broken at the welded joints between the orthogonal bars of the protective cage.
In an attempt to increase the resistance of the cage, and particularly to reduce the risk of failure at the welded joints between the bars, the prior art, in EP 1289852 suggested to form indents in the bars, directly adjacent to the welded joints, at a distance from the bar intersection equal to one tenth of the width of the bars.
This arrangement is aimed at forming higher flexibility points in the cage bars, to reduce stresses at the welded joints between the bars.
Thus, the cage structure so obtained is more flexible.
Furthermore, according to EP 1289852, the walls of the two opposing bars at the welded joints must be spaced to some extent, to allow quick drying of still water in the gaps, and prevent rust formation.
Still according to EP 1289852, the bars may have not more than 2 mm deep recesses in the welded joints. This restriction, when combined with the lack of contact between the bar walls, prevents achievement of the maximum achievable resistance at the welded joints.
Another pallet container is disclosed in U.S. Pat. No. 5,678,688, which addresses the problem of increasing the durability of the welded joints between the tubular bars of the outer cage that holds the plastic container therein. U.S. Pat. No. 5,678,688 suggests forming indents in the bars, directly adjacent to the intersections between the bars, to form bar bending points. Particularly, the indents are formed in a hollow area of the bar, where the welded joints are formed.